Conventional transformer coupled plasma (TCP) processing chambers utilize a source coil disposed outside of the chamber and above an upper window of the chamber. In this configuration, the source coil is separated from the plasma processing environment within the chamber. Also in this configuration, radiofrequency (RF) power is supplied to the source coil to generate an RF electromagnetic field that must extend through the window and into the chamber to strike and sustain a plasma within the chamber. A large RF current must be supplied to the source coil above the window in order to generate an RF electromagnetic field within the chamber that is sufficient to strike and sustain the plasma. However, use of such large RF currents can have side-effects such as creation of high-intensity electric fields, capacitive coupling, and corresponding ion bombardment on the window, which can cause erosion of the window.
Also, the position of the source coil outside of the chamber and above the window causes the source coil to be positioned at a relatively remote location from the substrate to be processed. The remote location of the source coil from the substrate can cause a reduction in the achievable plasma density at the substrate level due to the relatively large diffusion distance for neutral species and ions within the plasma. Also, the window configuration can cause a reduction/limitation of the anode area of the chamber, which can cause a decrease in the anode-to-cathode area ratio of the chamber, which in turn can result in higher than desirable plasma potential. Additionally, use of the window itself introduces complexity to the chamber in that the window needs to achieve a high temperature through direct heating or through process-induced heating in order to keep the chamber in equilibrium, so as to avoid “first wafer effects.”
In addition to the above, as the substrate size increases, the plasma chamber size is required to increase in a corresponding manner. Increasing the chamber size requires an increase in the window size. Because the window must be capable of withstanding a high pressure differential between the inside of the chamber at vacuum pressure and the outside of the chamber at atmospheric pressure, an increase in the window size requires an increase in the window thickness. The increased window thickness will require the source coil positioned outside of the chamber and above the window to be excited with even higher RF currents and power levels to obtain a sufficient energy transfer to the plasma within the increased size chamber. Thus, the issues noted above with regard to high RF currents may become even more pronounced with the increased size chamber.